1. Field of the Invention
This invention relates to a chip varistor and its production method and is particularly directed to a chip varistor and its production method having an improved structure of electrodes so as to obtain high surge current endurance.
2. Description of the Prior Art
An increase in the amount of varistors, which are non-linear resistance elements, currently in use corresponds to the increased use of electric machines and devices in recent years. In addition, chip varistors are increasingly needed due to miniaturization of devices.
An important feature of chip varistors is surge current endurance. In use, chip varistors protect against both large and small current surges. Therefore, chip varistors are critical components with respect to product reliability and durability.
FIG. 1 shows a cross sectional view of a conventional chip varistor 20.
The chip varistor 20 is comprised of: a rectangular-parallelepiped shaped varistor element 21 which is mainly made of zinc oxide (ZnO); and, electrodes 22a, 22b made of a material in which boro-silicated glass frit is added to, for example, silver. The electrodes 22a, 22b are in contact with the varistor element 21. One electrode extends from one opposing surface to one end of the varistor element 21 and the other electrode extends from the other opposing surface to the other end of the varistor element 21.
Application of a certain voltage to the pair of electrodes 22a, 22b and a large amount of current to the varistor element 21 causes an intensified concentration of electric current at ends of the electrodes 22a, 22b. This intensified concentration of electric current is indicated by the arrows shown in FIG. 2. In addition, the sections of the varistor element 21 proximate to the ends of the electrodes 22a, 22b are also damaged, as a result of the above mentioned condition. Thus, the surge current endurance of the varistor 21 is decreased.
Furthermore, gap intervals "b" between the electrodes must be larger than the thickness "a" of the varistor element, as shown in FIG. 3. Therefore, this creates a dimensional limitation in producing electric poles.
Hence, it is an object of the present invention to provide a new chip varistor which has a greater surge current endurance than conventional chip varistors, thereby increasing the reliability and durability of devices utilizing the new chip varistor, and an improved method of manufacturing chip varistors.